The present invention relates to methods and apparatus for the detection of halogens, in particular fluorine.
The processing steps of silicon wafers for the manufacture of semiconductors use a wide range of precursor gases.
Precursor gases have very short residence times in a process chamber. Consequently, much of the gas is unused and any remaining process chemicals or their by-products are conveyed from the chamber by vacuum pumps to abatement equipment were they are destroyed to prevent their release into the environment.
Chamber cleaning processes and other wafer etch processing steps use gases such as NF3, SF6, perfluoroalkanes (PFCs) and Fluorine. These gases are either activated remotely and then passed to the chamber, or activated within the chamber, to produce fluorine radicals which etch silicon based deposits from the chamber walls or the surface of the wafer. As well as the reaction of the radicals with silicon oxide a certain percentage will also recombine to form “sink” PFC compounds, such as CF4, or react to form diatomic fluorine molecules. These reaction by-products need to be destroyed due to their respective high global warming potential and toxicity.
When carrying out abatement of gases, such as those exhausted from an etch process, post abatement equipment exhaust gas analysis is often needed to ensure that the equipment is working properly and that each of the gases is being destroyed to below allowable legal limits.
Gases such as PFCs are easily measured and monitored using techniques such as infrared spectrometry, gas chromatography and continuous flow mass spectrometry. However there are several problems determining the presence of fluorine, and other halogens, in the exhaust gases with these techniques.
Due to their homonuclear diatomic structure the stretching vibration of the bond in the halogens F2, Cl2 and Br2 does not cause a change in dipole moment and as such they are not detectable by infra red spectroscopy.
Due to the corrosive nature of the halogens, techniques such as gas chromatography require costly specialist columns which may not be suitable for the simultaneous detection of PFCs.
Similarly, cross sensitivity of mass spectrometers to other gases often present in semiconductor exhaust streams, such as argon and water vapour, interferes with the measurement of fluorine. In addition, prolonged exposure to corrosive gases such as halogens can often damage the delicate spectrometer instrumentation.
JP 63-27736 describes a method of passing a fluorine containing gas stream through a column of sulphur to convert the fluorine to SF6, which is then analysed by infrared spectroscopy. However, by this method the user is not able to determine whether all the fluorine has been converted to SF6. In order to be confident that the complete conversion of the fluorine had occurred a user would require either a long reaction column or very fine sulphur, which would cause problems with the conductance of the gas stream through the column.
Another example is that described in JP 63-247655 in which a gas stream containing fluorine is first passed through a column of potassium chloride to form a gas stream containing hydrochloric acid, which is subsequently passed through a column of potassium iodide to form a gas stream containing iodine. The liberated iodine can then be optically analysed. However, this technique is laborious and expensive, requiring two conversion steps, and in addition it is not possible to ensure that all the fluorine in the gas stream has been converted.
A further method of detecting the concentration of fluorine is that described in US20020051132 in which an exhaust gas stream containing fluorine and a hydrofluorocarbon (HFC) gas are passed through a solution containing a metal iodide. The fluorine reacts with the metal iodide to liberate iodine which is then analysed using light in the 460 nm to 520 nm region.